The present invention relates to a method of manufacturing cathode ray tubes and to cathode ray tubes made by the method, the cathode ray tubes having a multilayer interferenece filter disposed between the cathodoluminescent display screen and the interior side of the faceplate. Such cathode ray tubes may comprise projection television tubes.
The present invention also relates to a projection television system comprising three cathode ray tubes having cathodoluminescent screens luminescing in different colours, wherein at least one of said cathode ray tubes comprises a tube made in accordance with the present invention.
A multilayer interference filter comprises a number of layers manufactured alternately from a material having a high refractive index and a material having a low refractive index. Projection display tubes including such multilayer interference filters are disclosed in European Patent Publication 0170320 (PHN 11.106), unpublished Netherlands Patent Application 8502226 (PHN 11.460) and unpublished British Patent Application 8513558 (PHQ 85.007). Typically the alternate layers may comprise in the case of a low refractive index material SiO.sub.2 (refractive index n=1.47) or MgF.sub.2 (n=1.38) and in the case of a high refractive index material TiO.sub.2 (n=2.35) or Ta.sub.2 O.sub.5 (n=2.00) the precise value of n being dependent on the substrate temperature during evaporation and also on the annealing cycle after evaporation. These known multilayer filters comprise at least six but more typically at least fourteen layers alternately made from the respective high and low refractive index materials. The layers have an optical thickness nd, where n is the refractive index of the material of the layer and d is the thickness, the optical thickness nd of the individual layers being between 0.2.lambda..sub.f and 0.3.lambda..sub.f, where .lambda..sub.f is equal to p.times..lambda., and .lambda. is the desired central wavelength selected from the spectrum emitted by the luminescent material of the relevant display screen, and p is a number between 1.18 and 1.32 for curved faceplates and between 1.18 and 1.36 for flat faceplates. The average optical thickness throughout the stack, excluding possible outer terminating 0.125 .lambda..sub.f layers, is 0.25.lambda..sub.f and .lambda..sub.f is the central wavelength of the filter. Although these so-called shortwave pass multilayer interference filters perform reasonably satisfactorily, further investigation has shown that the filters can suffer from crazing (formation of cracks) after the tube processing is completed. The crazing manifests itself, subsequent to the evaporation of the filter layers, after tube processing which includes temperature cycles up to 400.degree. to 460.degree. C. Such crazing reduces the quality of the optical performance of the multilayer interference filter.
A letter entitled "Observation of Exceptional Temperature Humidity in Multilayer Filter Coatings" by Peter Martin, Walter Pawlewicz, David Coult and Joseph Jones, published in Applied Optics, Vol. 23, No. 9May 1, 1984, pages 1307 and 1308, discloses multilayer filter coatings made by reactive sputtering techniques using Si.sub.3 N.sub.4 /SiO.sub.2 and Nb.sub.2 O.sub.5 /SiO.sub.2 as the high and low refractive-index layers. The design of the Si.sub.3 N.sub.4 /SiO.sub.2 filter was LL(HL).sup.14 HLL where L and H represent a quarterwave optical thickness of low-and high-refractive index material, respectively, whereas the design of the Nb.sub.2 O.sub.5 /SiO.sub.2 filter was LL(HL).sup.10 LL. This letter reports that temperature and relative humidity testing with temperatures in the range of 75.degree. C. to 140.degree. C. and relative humidities between 0 and 85% indicated that as far as transmittance in the sidebands is concerned, a Si.sub.3 N.sub.4 /SiO.sub.2 coating was remarkably more stable than a Nb.sub.2 O.sub.5 /SiO.sub.2 coating. This letter does not provide details of how each multilayer filter is made, especially the nature of the substrates, the deposition temperatures and subsequent processing of the filter, all of which have some bearing on the crazing, the quality of bonding between, and the hardness of, the layers and the actual refractive indices of the material. Furthermore the authors of this letter have not addressed themselves to the provision of interference filters in cathode ray tubes where the problems are different because amongst other things: 1. the much higher temperatures, above 400.degree. C., used in tube processing (crazing has been found to be initiated above about 330.degree. C.); and 2. the electron bombardment during tube operation.
An object of the present invention is to reduce and preferably avoid crazing in multilayer interference filters used in cathode ray tubes.
Another object of the present invention is to reduce the cycle time for filter evaporation.
According to a first aspect of the present invention there is provided a method of making a cathode ray tube having a multilayer interference filter provided on an internally facing surface of a faceplate, the method including the step of depositing alternate layers of a material having a relatively high refractive index and a material having a relatively low refractive index on the faceplate, the material having a relatively high refractive index comprising niobium pentoxide.
According to a second aspect of the present invention there is provided a cathode ray tube having a faceplate, a cathodoluminescent screen and a multilayer interference filter disposed between the faceplate and the screen, the filter comprising alternate layers of a material having a relatively high refractive index and a material having a relatively low refractive index deposited on the faceplate, wherein the material having a relatively high refractive index comprises niobium pentoxide.
The advantages of using niobium pentoxide compared with titanium dioxide are firstly that it can be evaporated at a much lower temperature, 80.degree. C. for niobium pentoxide as compared to 300.degree. C. for titanium dioxide, which reduces the cycle time by about a factor of two, and secondly that the resulting filters with niobium pentoxide are more resistant to crazing when subjected to a heating cycle including temperatures up to 400.degree. to 460.degree. C., which heating cycle is necessary in processing the completed faceplate.
When titanium dioxide is evaporated at lower temperatures the oxidation is slowed down appreciably, resulting in either not fully oxidized and therefore light absorbing layers or unacceptably long evaporation times and lower refractive indices of the layers. Niobium pentoxide can be evaporated with a high rate at a temperature as low as 80.degree. C., yielding layers with a high refractive index. Such a high rate of evaporation of niobium pentoxide at 80.degree. C. reduces the cycle time for filter evaporation.
The advantages of using niobium pentoxide compared with tantalum pentoxide are firstly that niobium pentoxide has a substantially higher refractive index, yielding filters with a much broader reflection band, and secondly that the interference filters with niobium pentoxide are more resistant to crazing when subjected to the heating cycle including temperatures of up to 400.degree. to 460.degree. C.
One embodiment of a filter comprised niobium pentoxide as the high refractive index material and silicon dioxide as the low refractive index material. 20-layer Nb.sub.2 O.sub.5 /SiO.sub.2 filters evaporated with substrate temperatures of 80.degree., 200.degree. and 300.degree. C., had little or no crazing after being heated to temperatures of 460.degree. C. The reason for this unexpected result is that tests with: (1) 20 layer TiO.sub.2 /SiO.sub.2 filters evaporated with substrate temperatures of 300.degree. and 400.degree. C., (2) 20 layer Ta.sub.2 O.sub.5 /SiO.sub.2 filters evaporated with substrate temperatures of 80.degree. and 200.degree. C., and (3) (10/4).lambda..sub.f SiO.sub.2 layers, that is layers having an equivalent thickness of SiO.sub.2 as in the filters in (1) and (2) above, evaporated also with different substrate temperatures, all showed more and a mutually very similar amount of crazing when subjected to the same temperature cycling with temperatures of up to 460.degree. C. Interleaving silicon dioxide with niobium pentoxide reduces the occurrence of crazing, in some cases even to such an extent that it no longer occurs. These comparative tests were performed using as substrate material, projection television faceplate glass having an expansion coefficient of 95.times.10.sup.-7.
In another embodiment the filter comprised niobium pentoxide as the high refractive index material and magnesium fluoride, as the low refractive index material. 20-layer filters of these materials evaporated with substrate temperatures of 200.degree. and 300.degree. C. did not show any crazing.
The cathode ray tube made in accordance with the present invention may comprise at least 9 layers, typically between 14 and 30 layers, each layer having an optical thickness nd, where n is the refractive index of the material, d is the thickness. The optical thickness nd is chosen to lie between 0.2.lambda..sub.f and 0.3.lambda..sub.f, more particularly between 0.23.lambda..sub.f and 0.27.lambda..sub.f, with an average optical thickness 0.25.lambda..sub.f, where .lambda..sub.f is equal to p.times..lambda., where .lambda. is the desired central wavelength selected from the spectrum emitted by the cathodoluminescent screen material and p is a number between 1.20 and 1.33.
The faceplate may comprise a mixed-alkali glass substantially free of lead oxide having a coefficient of expansion in the range from 85.times.10.sup.-7 to 105.times.10.sup.-7 per degree C. for temperatures between 0.degree. and 400.degree. C. The main components in weight percent of such a glass may be
______________________________________ SiO.sub.2 50 to 65 Al.sub.2 O.sub.3 0 to 4 BaO 0,5 to 15 SrO 8 to 22 K.sub.2 O 3 to 11 Na.sub.2 O 3 to 9 Li.sub.2 O 0 to 4 ______________________________________
with the restrictions that (1) BaO and SrO together lie between 16 and 24, and (2) the combination formed by Li.sub.2 O, Na.sub.2 O and K.sub.2 O lie between 14 and 17.